Despite significant advances in our understanding of many of the peripheral effects of activating the medial olivocochlera (MOC) efferent fibers to outer hair cells (OHCs), and of the molecular mechanisms underlying those effects, the functional significance of this pathway in human audition remains controversial. In both awake humans and anesthetized animals, the MOC fibers respond to sound, either ipsilaterally or contralaterally delivered, and thus constitute the effector limb of a binaural sound-evoked reflex pathway. However, we don~t know if the basic organizational plan of the reflexes seen in animals applies to human. When activated by sound, MOC fibers alter the state of the OHCs and thereby affect the generation of a variety of otacoustic emissions (OAEs). This phenomenon provides a non-invasive way to examine the activation levels of the MOC pathway. Our approach is to use OAEs to assay MOC activation levels in normal human subjects, and to compare the levels of activation in a variety of listening contexts. We have developed OAE-based methods that allow measurement of MOC activation from any source, including ipsilateral activation which has been difficult to measure in the past. To understand the functional organization of the human MOC system, we will, in humans (Aim 1a) determine the relative strength of the ipsilateral, contralateral and binaural MOC reflexes, (Aim 1b) determine the frequency resolution of the MOC reflexes and (Aim 1c) determine MOC reflex strength across frequency in terms of the underlying changes in cochlear sensitivity. To address the functional significance of the MOC pathway, we will (Aim 2a) measure MOC activation levels in psychophysical tasks in which we predict that MOC activation should aid performance (discrimination of transient signals in noise) and those in which no benefit is anticipated, and (Aim 2b) measure MOC activation levels during selective attention to auditory vs. Visual stimuli or to one component of a complex auditory stimulus. The results of our experiments will provide the first comprehensive assessment of the fractional organization of the human MOC system and should provide significant insight into the functional significance of the MOC system in humans.